Our parents have told us that millions of times. But do we really listen? We should.
My friend almost had a shidduch pulled out from under her feet because of some sick rumors about the boy. I can't go into any details but because of some person who decided that somehow the situation justified exaggeration or making up some "facts", she almost did not go out with this guy again even tho she really liked him.
you have to be so careful with things you say, especially when it comes to shidduchim. but in all areas as well. with shidduchim, people can say nasty things just because they don't like your parents, the school you went to, or because they're just plain jealous. make sure to get sources for everything you hear and don't believe something until it has been absolutely proven! it's not fair to either side. it wasn't fair to my friend. it wasn't fair to the boy.
we're dealing with people's lives here.
i remember a few years ago, d'rav meir put on a play that was publicized by CCHF because it was about a girl who said lashon hara about another girl and it RUINED HER LIFE. i don't remember the story but it was really powerful.